This invention relates to an oscillation circuit for providing an oscillation output having a duty ratio different from 50%, and more particularly to an oscillation circuit for controlling, for example, a bootstrap circuit included in an EPROM (Erasable Programmable Read Only Memory) which is erasable by means of ultraviolet rays.
In general, a package for mounting an EPROM which is erasable by means of ultraviolet rays is provided with a window for allowing ultraviolet rays to be radiated on the EPROM. During erasing of the stored contents, ultraviolet rays are radiated on the EPROM through the window. However, during writing of data in the EPROM, external light rays may also penetrate through the window to radiate on the EPROM, so that the light rays incident on junction capacitances in a bootstrap circuit included in the EPROM may cause leakage of charges stored in the junction capacitance, resulting in that the write-in voltage is lowered to a level insufficient to write in data. In order to avoid this lowering of the write-in voltage, an oscillation circuit is usually included in the EPROM, so that the junction capacitances are repeatedly charged up during the writing.
In a conventional oscillation circuit, the output signal has a duty ratio substantially equal to 50%. Therefore, within one period of the output frequency, the interval of a high potential is substantially equal to the inteval of a low potential. When the output of this oscillation circuit is connected to the input of the bootstrap circuit in the EPROM, the high potential interval and the low potential interval usually correspond to a programmable interval and a reset interval, respectively, as hereinafter described in detail. Accordingly, by the use of the conventional oscillation circuit, the programmable interval, or in other words, the write-in interval, is substantially equal to the reset interval, or, the interval for charging junction capacitances included in the bootstrap circuit.
However, for the purpose of increasing the write-in efficiency, the write-in interval is required to be as long as possible and the reset interval is required to be as short as possible. To this end, the input of the bootstrap circuit should preferably receive an oscillation signal having a duty ratio different from 50%. To obtain such a signal having a duty ratio different from 50%, a combination of frequency dividers is connected to the output of the conventional oscillation circuit. However, such a prior art technique has disadvantages in that a number of frequency dividers are required, causing the cost to be increased as well as necessitating the troublesome task of making frequency dividers for obtaining the desired duty ratio.